A conventional control device having a forced operation function will be described with reference to FIG. 3. FIG. 3 is a circuit diagram showing, in particular, that portion of the control device which operates in response to an output control signal S1. The output control signal S1 is, for example, an interruption signal as generated by a control circuit (not shown). The output control signal S1 is fed through an OR circuit 1 to a number n of output circuits C1-Cn.
The output circuits C1-Cn are, for example, buffer circuits, delay circuits, or triangular wave generating circuits. There are provided as many output circuits C1-Cn as are required by a personal computer or the like that is connected to terminals T1-Tn so as to be controlled by the control device. In response to the output control signal S1, the output circuits C1-Cn generate signals, such as signals delayed by a predetermined length of time or triangular waves, and feed those signals out via the terminals T1-Tn.
To make testing of the operation of the output circuits C1-Cn possible, the control device is provided with a switch terminal T0. The switch terminal T0 is connected to the non-inverting input terminal (+) of a comparator 2. At its inverting input terminal (-), the comparator 2 receives a voltage higher by a predetermined voltage Vref than the ground level.
When a voltage higher than the voltage Vref is applied to the terminal T0, the comparator 2 outputs a high level and thereby generates a forced operation signal S2. The output control signal S1 and the forced operation signal S2 are fed to an OR circuit 1, and therefore, as long as the OR circuit 1 is receiving at least one of those two signals, it outputs a high level. This makes it possible to operate the output circuits C1-Cn forcibly and thereby test their operation.
However, this conventional control device (FIG. 3) requires provision of an extra terminal, specifically the switch terminal T0. Moreover, to provide the switch terminal T0, it is inevitable to provide an extra wire pad (i.e. an electrode provided on a chip for connection with a wire). This leads to an increase in the chip size and thus in the manufacturing costs. For this reason, in ICs (integrated circuits) having a relatively few number of terminals, it is impossible to provide a switch terminal, and thus the control devices provided in such ICs cannot be operated forcibly.